1) Field of the Invention
This invention relates to a multilayer, plastic, easy-opening closure for hermetic sealing of an open end of a retortable container.
2) Description of Related Art
A variety of closures are known for the hermetic sealing of a container, such as conventional tin-plated steel cans that are widely used for containing food products. Retortable containers are those that can withstand a pasteurization or retort process comprising heat and pressure for preserving the food contents of the container. During retort, the container can be subjected to temperatures above 212° F. and up to 250° F. under pressures of 15 to 30 psi.
Easy-opening containers are those that can be opened without undue effort and without the use of a special tool such as a rotary can opener. In order for an easy-opening container to be retortable, the closure must be sufficiently strong to resist stresses that develop as a result of the retort heat and pressure, and yet the bond between the closure and container must be easily overcome during opening. One conventional easy-opening, retortable container includes a closure that is stronger in shear than tension. The closure is strong enough to withstand the shear force that develops during retort, while a relatively small tensile force is required to open the container. For example, U.S. Pat. No. 5,752,614, titled “Easy-Opening Closure for Hermetic Sealing a Retortable Container,” to Nelson describes an easy-opening closure that includes a metal end ring that can be seamed to an open end of a retortable container and defines a central opening that is covered by a membrane patch. The membrane patch is bonded to the end ring such that the bond is unaffected during retort processing but has a predetermined tensile force strength that is preferably less than 14 pounds to allow peeling of the membrane patch from the end ring. Thus, the container can be retorted and subsequently easily opened.
Other easy open containers are formed partially or wholly of plastic. For example, a lid that includes a metallic foil can be adhered to a plastic container, which can be used for retort. The adhesive used to bond the foil to the container can be a glue bond that is strong enough to withstand retort but easily overcome when a consumer or other user peels the foil from the container. Attempts have also been made to manufacture retortable containers exclusively of plastic. For example, a plastic membrane can be heat-sealed to a plastic container. Alternatively, the plastic membrane can be heat-sealed to a plastic end ring that is similar to the metal end ring described by Nelson, and the plastic end ring can be fusion bonded to the container by spin welding. Such bonds can be difficult to form by heat-sealing due to the poor thermal conductivity of the plastic. If the strength of the bond is not controlled precisely, the bond may be insufficiently strong to resist the stresses that are applied during retort or too strong for a user to overcome in order to open the container. Further, the heat required for heat-sealing can affect the color or the contour of the plastic membrane, thereby detracting from the aesthetic appeal of the container.
Thus, there exists a need for a plastic, easy-opening closure for hermetically sealing an open end of a retortable container and an easy-opening container that is hermetically sealed by such a closure. The closure should be strong enough to withstand the stresses induced during retort, but easily removed by a user.